The present invention relates to an electrophotographic recorder such as a printer, facsimile, and copier for visualizing an image using colored particles such as toner and more particularly to a process of forming a full-color toner image on the surface of a recording medium.
A conventional developing method and developing machine will be explained hereunder. A recorder using the electrophotographic system is composed of a printing process for visualizing colored particles as an image on the surface of a recording medium and a fixing process for fixing the visualized colored particle image onto the recording medium. For colored particles, powder called toner used only for electrophotograpy is used. A photoconductor is charged overall the surface thereof once and then by irradiating light, the surface is partially discharged. In this case, on the surface of the photoconductor, a potential contrast due to the charging region and discharging region is formed and this is called an electrostatic latent image. In the developing process, firstly toner particles which are colored particles are charged. Toner particles are conveyed to the developing position opposite to the electrostatic latent image on the surface of the photoconductor by a roller called a developing roller. As a developing method for an electrostatic latent image, a method called bias development is often used. In the bias development, the bias voltage is applied to the developing roller and by the operation of the electric field generated between the latent image potential formed on the surface of the photoconductor and the developing roller, charged particles are separated from the surface of the developing roller, moved to the surface of the photoconductor, and imaged. As a latent image potential (namely, potential of the image formed portion of the photoconductor), the aforementioned charging potential or discharging potential may be used. Generally, a method for using the charging potential as a latent image potential is called a normal developing method and a method for using the discharging potential is called a reversal developing method. The potential among the charging potential and discharging potential which is not used as a latent image potential is called a background potential. The bias voltage of the developing roller is set between the charging potential and the discharging potential and the difference from the latent image potential is called a developing potential difference. In the same way, the difference from the background potential is called a background potential difference.
As a developed form of the printing process using the aforementioned bias development, there is a recording method available that the bias development is executed using primary color toner, and the primary color toner is transferred onto a recording medium such as a recording paper or an intermediate transfer medium, and this process is repeated sequentially on one recording medium for each primary color, thus a full-color image having a medium color is obtained. This method is called a tandem color process. To superimpose toners and represent a mediate color, subtraction color mixing using three primary colors of cyan (C), magenta (M), and yellow (Y) is used. Black (K) conspicuous in unequal color tone at the time of color mixing is often added as a primary color to form four primary colors. To use addition color mixing for requiring no superimposition of the same primary colors and representing a medium color by putting them side by side is possible in principle. However, in a case of a toner image by the electrophotographic method, the light source must depend on reflection of extraneous light, so that when addition color mixing is used, the saturation is reduced, and the color reproduction range is made extremely small, thus the addition color mixing method cannot be put into practical use. Therefore, in the tandem color process, the positioning accuracy between the primary colors is a very important factor for medium color reproduction by the subtraction color mixing method. Generally, it is desired that the displacement between the pixels of each primary color of C, M, and Y on the final recording medium, that is, the output image is 40 microns or less between any two pixels. However, practically, such accurate positioning (color superimposition) is technically difficult considerably and even in this case, it is said that the displacement must be 100 microns or less. By this method, only one color can be developed for one photoconductor device and to reproduce a medium color using the three primary colors and black, four independent printing processes are necessary for each primary color, so that a problem arises that the recorder is apt to be made larger.
Further, to reproduce a medium color using the three primary colors, two times of positioning is required and a problem also arises that the color superimposition accuracy between the primary colors cannot be increased easily.
Furthermore, when a primary color other than the primary colors C, M, and Y is added to reproduce a medium color, it results in enlargement of the recorder as it is, so that a problem arises that the color reproduction range is limited to the range obtained by subtraction color mixing of the three primary colors and becomes narrower.
As the aforementioned tandem color process, for example, the methods described in Lucien A. De Schamphelaere and Xeikon Team, xe2x80x9cDigital Color Presses Applications and Technologiesxe2x80x9d, ISandT""s 10th International Congress Proceedings, pp. 517-526 (1994) and ISBN: 0-89208-179-1 are known.
In the aforementioned conventional full-color recorder, an independent printing process is necessary for each primary color, so that there are problems imposed that the recorder is apt to be made larger and the color superimposition accuracy between the primary colors cannot be easily improved. Furthermore, a problem arises that the color reproduction range is limited to the range of subtraction color mixing of the three primary colors and becomes narrower.
Therefore, an object of the present invention is to provide a full-color recorder excellent in color reproducibility because it is miniature and color superimposition accuracy can be obtained easily. Furthermore, another object of the present invention is to provide a full-color recorder having a wider color reproduction range beyond the range of subtraction color mixing of the three primary colors.
The above objects can be accomplished by using two potential split developing processes.
Next, the potential split developing processes will be explained. As a deformation example of the electrophotographic bias developing method, for example, as seen in Japanese Application Patent Laid-Open Publication No. Sho 48-37148 for long, there is a developing method that the potential of the charging region and discharging region of the photoconductor is divided into two parts and an intermediate potential region is provided, and a first developing unit for executing normal development is installed in the discharging region and develops the first toner, and then a second developing unit for executing reversal development is installed in the charging region and develops the second toner, thus two kinds of toners are developed in one charging process and light irradiation process (exposure process). By this developing method, toner is not developed on the intermediate potential region (called intermediate potential) on the photoconductor having a voltage between the bias voltage of the first normal developing unit and the bias voltage of the second reversal developing unit and the background portion is formed as an image, so that an image by two kinds of toners composed of the background portion, first image portion, and second image portion can be formed. The developing method is called a potential split developing process. In the potential split developing process, two kinds of toners are generally used for each color and used for the purpose of obtaining an image composed of two colors. The background portion, first image portion, and second image portion are divided into regions depending on the potential level of the surface of the photoconductor. Therefore, these regions are formed without being superimposed. This indicates that the two kinds of toners are not mixed. By this method, a different color tone is not generated by mixing two kinds of toners, so that for the color tone, the color itself appearing in an image is used. By this method, for one photoconductor device, a two-color image can be recorded at a time, so that there is an advantage that the recorder can be made compact. Further, although two toners cannot be mixed, at a predetermined position between two-color images, an electrostatic latent image is formed by one exposure process, so that there is also an advantage that it is not displaced in principle. Even if the first development is set to the reversal development and the second development is set to normal development, the potential split development is enabled.
To realize a full-color recorder using the potential split developing process, in one recording medium, the first and second potential split developing processes are continuously arranged. For the first potential split developing process, two primary colors for subtraction color mixing are used and for one color of the subsequent second potential split developing process, the residual primary color is used. In this state, the two primary colors used in the first potential split developing process cannot be mixed, so that the developing color tone of the residual one color of the second potential split developing process is decided so as to compensate for mixing of the two primary colors. Concretely, when either of the two primary colors used for the first potential split developing process is used, the three primary colors can be mixed. Further, when a color other than those is used, the color reproduction range may be enlarged. Interchanging the first and second potential split developing processes or interchanging the first color development and second color development of the same potential split developing process does not adversely affect the color reproducibility, so that the effect of the present invention can be obtained in the same way.
By the aforementioned method, a medium color can be reproduced by two printing processes, so that compared with the conventional tandem color process, a miniature full-color recorder can be realized. Further, one positioning is sufficient to execute, so that the color superimposition accuracy among the primary colors can be improved easily. Furthermore, for the color tone of one of the first and second potential split developing processes for compensating for color mixing, a color other than the three primary colors may be used, so that a wider color reproduction range beyond the range of subtraction color mixing of the three primary colors can be realized.